The present invention relates to image sensors, in particular to image sensors which use a photo-sensing element array and matrix wires, and methods of manufacturing same.
One form of a long-length image sensor such as the contact type image sensors, makes use of the photoconductive effect. FIG. 1 is an illustration of such an image sensor. On a glass substrate 1, CdS.sub.0.2 Se.sub.0.8 :Cu layers 2 are arranged in a line shape. Then, electrodes 3, 4 facing each other and wires 5 are made from NiCr-Au layers. The electrodes 3 are formed as common electrodes for each group. After that, a film lead 6 is attached. The number 7 denotes polyimide films.
A fine pattern is difficult to produce on this structure because of the film lead 6, and the inability to accurately attach some, so that this technology is not appropriate to accomplish image sensors having a small width.
FIG. 2 is a diagram showing an image sensor made by the present inventor prior to the instant invention. In FIG. 2, there is illustrated an insulating substrate 21, on which is formed a semiconductor layer 22 such as a hydrogenated amorphous silicon (a-Si:H) layer and opposing electrodes 23 and 24. The electrodes 24 which have portions 24, are common electrodes for each photo-sensing element. The opposing electrodes 23 extend to form second wires 25. The second wires 25 respectively connect to first wires 26 which are formed parallel to the direction of disposition of the array of elements at the through holes C that are formed at the intersections of the first and second wires 25, 26 in the insulating layer 27 (for example, polyimide, epoxy, or the like).
When light reflected from the surface of a manuscript is to impinge upon the photo-sensing array, the electrical resistance of the elements changes in response to the amount of light incident upon the array. Thus, the image on the manuscript can be read, for example, by successively applying a voltage to the common electrodes 24 and by reading the changes in the resistance as electrical signals through successive scanning of the first wires 26.
However, an image sensor of this configuration has a number of disadvantages as discussed below.
The semiconductor layer 22 is generally formed in band form in the photo-sensing region by a masked plasma CVD method or the like. However, the amount of protrusion in the direction of the width of the semiconductor layer 22 has an unevenness due to nonuniformity of the contact between the mask and the substrate. Since the film thickness at the edge of the protrusion in the semiconductor layer 22 is nonuniform, if the insulating layer 28 is formed in that section in an overlapping manner, the adhesion of the insulating layer 28 is unstable. Therefore, the separation between the semiconductor layer 22 and the insulating layer 28 has to be made sufficiently large in order to avoid overlapping of the insulating layer 28 at the edge section of the semiconductor layer 22, which leads to a large size of the image sensor.
In order to eliminate the above problem, it is conceivable to spread the semiconductor layer 22 on the entire surface of the substrate 21, and subsequently remove areas except for the narrow band photo-sensing region utilizing, for example, a photoresist/etching technique. However, the etching of the semiconductor layer 22 involves the processes of coating of photoresist, exposure, and development of a resist and then removal of the resist, so that these steps become factors resulting in an increase in cost. Moreover, yield will also be reduced due to an increase in the number of required processes.
Furthermore, if the semiconductor layer 22 is first formed, the semiconductor layer 22 will be altered by the action of the etchant in the etching process for the insulating layer 28.
On the other hand, if the semiconductor layer 22 is formed after the formation of the insulating layer 28, it is difficult to form the semiconductor layer 22 with high quality due to the influence of the material for the insulating layer 28. In addition, there is a problem of damaging the insulating layer 28 caused by cleansing solutions such as a mixture of hydrogen peroxide and ammonia which is used for cleansing as a pre-treatment to the formation of the semiconductor layer 22.
Furthermore, in the above approach, the photoconductive layer and the insulating layer are made by different steps. However, it is desired to reduce the manufacturing steps for accomplishing high yield and lower cost.
Moreover, in the structure of FIG. 2, it is preferable to form additionally a protective insulating film on the photo-sensing element array, leading to a further increase in the number of manufacturing steps.